Modification of material surfaces plays a central role in modern chemical, biological and material sciences, as well as in applied sciences, engineering and technology. Methods for the modification of bulk material substrates have been developed by interfacial chemistry using organothiol-metals, enediol-oxides, silane-oxides, and other physicochemical methods, in which the predominant purpose is to impose desired properties on non-functional substrates. Molecules utilized for surface modification mostly have bifunctional end groups in which one end anchors to substrates and the other end provides chemical functionality to the substrate surface.
The existing toolbox for functional modification of material/substrate surfaces includes methods such as self-assembled monolayer (SAM) formation, functionalized silanes, Langmuir-Blodgett deposition, layer-by-layer assembly, and genetically-engineered surface-binding peptides. Although widely implemented in research, these conventional methods have limitations for widespread practical use. For instance, chemical specificity between interfacial modifiers and substrates (e.g., alkanethiols on noble metals and silanes on oxides) and complex instrumentation are typically required. In addition, the substrate size/shape (Langmuir-Blodgett deposition) is often limited, or multi-step procedures for implementation (layer-by-layer assembly and surface-binding genetically engineered peptides) are required. Further, existing compounds are often expensive and/or difficult to use.
Methods comprising the single-step coating of substrates with active agents are known in the art. For example, dopamine is capable of spontaneously modifying a variety of substrate surfaces under oxidative conditions (see U.S. Pat. No. 8,541,060 to Messersmith et al.). Dopamine contains a primary amine that facilitates intramolecular cyclization to form the 5,6-dihydroxyindole intermediate that is essential for polydopamine formation. Thus, one of skill in the art would have no reasonable expectation that the method disclosed in the '060 patent would be successful using a nitrogen-free surface modifying agent. Furthermore, polydopamine coatings as described in '060, related patents, and in the academic literature are, without exception, dark colored coatings, as they are closely related to the chemical composition of melanin pigments. The dark color conferred by polydopamine coatings is problematic for many practical applications of the technology where masking or discoloration of the inherent substrate appearance is to be avoided for aesthetic or performance reasons.
In addition, coatings derived from natural sources are also known to the art. For example, tannic acid has been used to modify substrate surfaces (see Caruso et al. 2013). The Caruso art requires the use of trivalent metal ions (Fe3+, V3+, Gd3+, or Cr3+ ions) and relies on metal-oxygen coordination bonds formed between the tannic acid and trivalent metal ions for formation of the coating. They show that their coatings do not form in the absence of trivalent metal ions. Additionally, these coordination-based coatings require an acidic-to-basic pH adjustment to form. Furthermore, the Caruso art produces coatings that are darkly colored and are inherently unstable at pH values less than 7.0. Moreover, the coatings based on Caruso art utilize high concentrations of iron, which will likely be toxic to biological systems if biomedical applications are pursued. Coatings that incorporate tannic acid as one component of a multi-component coating have been formed by so-called layer-by-layer technology (see Shutava et al. 2005). However, layer-by-layer coatings involve multi-step deposition processes and require the use of other molecules for formation. In contrast, the present invention describes a general method requiring only a plant-based or plant-inspired phenol or polyphenol precursor compound deposited in a single step to yield colorless coatings that are stable over a wide range of pH conditions.
Accordingly, cost-effective and easy-to-use compounds and methods for the surface-independent modification of a substrate whereby specific functional moieties can be displayed on the surface are needed.